A conventional OFDM receiver has a demodulator which has to be tuned very accurately to the received signal. The receiver sampling clock must also be accurately aligned to the received signal. Normally, the demodulator carries out coarse frequency synchronisation (signal acquisition) and fine frequency tuning or tracking. The demodulator also performs coarse timing (OFDM symbol synchronisation) and fine sample clock synchronisation.
The present invention is primarily concerned with fine frequency tuning and fine sample clock synchronisation. Various arrangements are known for carrying out these operations in such a way as to minimise the effect of multipath interference, which can give rise to inter-symbol-interference (ISI). Some such arrangements rely upon each OFDM symbol comprising a “useful part” and a “guard space”, the guard space sometimes being referred to as a guard interval, cyclic extension or cyclic prefix. The guard space precedes the useful part of the symbol and contains a repeat of the data at the end of the useful part.
In some prior art arrangements, time-domain samples which are spaced apart by the useful symbol period are cross-correlated to achieve symbol synchronisation, fine frequency correction and sample clock adjustment. Other arrangements derive fine frequency and sampling timing control signals from the output of the conventionally-provided fast Fourier Transform (FFT) block which is provided for demodulation of the time-domain samples.
It would be desirable to provide an improved technique for dealing with fine frequency and/or sampling clock offsets.
According to the present invention an OFDM receiver has, in addition to an FFT block for extracting data from a received signal, a further means for performing a partial and/or reduced Fourier transform on received time domain samples. A “partial” FT is one in which only a subset of the frequency bins is calculated. A “reduced” FT is one which has a set of bins which is smaller than the set which is used to demodulate the OFDM signal, i.e. it is a short FT. The partial and/or reduced Fourier transform can derive a phase variation value, for a particular frequency output bin, which represents an error resulting from mis-tuning of the local oscillator frequency. Alternatively, or additionally, the contents of two output bins of the partial and/or reduced FFT car be evaluated to determine two separate phase variations, any difference between those phase variations resulting from an error in the sample clock frequency. Accordingly, the partial and/or reduced FT can provide signals for correcting the local oscillator frequency and/or the sample clock.
In the preferred embodiment, each phase variation represents the difference between the phase angles of the same output bin for different parts of the OFDM symbol, those different parts containing the same data. This is done by performing the partial and/or reduced FT on samples of the guard space and the matching samples in the useful part of the symbol. In this embodiment, because several samples in the guard space match corresponding samples in the useful part of the symbol, it is sufficient to perform a reduced FT on groups of samples. Only one or two bins need to be calculated.
In an alternative embodiment, which is particularly applicable to OFDM transmissions containing pilot signals of known value, each phase variation can represent the difference between the phase of the complex sample in a particular output bin and the expected phase of a pilot signal. In this embodiment, it is necessary to resolve an individual pilot symbol, so one or two frequency bins of a full FT are calculated.